1. Function and composition.

The fuel system is one of the important dynamical systems of diesel engine. The function of the fuel system is to transfer the fuel to the inlet of the injection pump unimpeded.
The system is usually composed of five basic links: loading and measurement, storage, transfer, purification treatment and supply.
The fuel loading is carried out by the fuel injection flanged joint on the deck of the ship.
Thus, light and heavy fuel can be injected directly into the tank from both sides.
Injection pipe should prevent overpressure facilities.
If the safety valve is used to prevent overpressure equipment, the overflow of the valve shall be discharged into the overflow tank or other safety premises.
The injection joint must be higher than the deck plane and sealed with a cover plate to prevent the water from being pumped into the tank during rough weather.
The measurement of fuel oil can be carried out through the measuring hole of each fuel tank. If the fuel tank is equipped with a sounding instrument, it can also be carried out through the sounding instrument and then the control deck.
The added fuel is stored in the fuel tank cabinet.
For the heavy oil tank, the heating coil is usually installed to heat the heavy oil to maintain its liquidity and facilitate the refueling.
 

The fuel system is also equipped with a transfer valve box and a refueling pump for refueling oil between each tank.
The fuel from the oil tank must be purified before entering the machine.
Fuel purifying systems include heating, precipitation, filtration and centrifugation of fuel.
Figure 5-1 shows the heavy fuel oil purification system used by most ships.
Through the barge chest 1, is fuel transfer pump from the tank into the sedimentation tank 5, each limiting filling oil level sensor, between 3 and 3 automatically adjust the steam flow of oil heating system to accelerate the precipitation separation and precipitation tank can make the offer for the oil pump 7 oil temperature variation is small.
A pneumatic constant pressure valve 9 and a flow control valve 9 are provided for the oil pump to ensure a smooth flow of fuel to the oil separator, which is conducive to improving the purification quality.
The heating temperature is controlled by the temperature controller 10, so that the fuel temperature of the oil separator is almost constant.
The system is equipped with a spare oil separator which can be connected in series with the main oil separator and can be connected in parallel. It also has a spare oil pump, which improves the reliability of the system.
The net oil of the oil separator enters the daily oil tank 15, and the daily oil tank sets the overflow pipe.
Under the condition of the normal sailing ship, oil purifier points of oil will be some larger than the consumption of diesel engine, so close to the entrance of the suction low daily service tank is equipped with the overflow pipe, can make the daily service tank bottom temperature is lower, the impurities and water content more fuel back into settling tank, both implementation cycle improve the separation effect, and make the oil purifier start-stop frequency decreases, prolong the service life of oil purifier.
Both the sedimentation tank and the daily cabinet are equipped with water level sensors 6 and 16 to remind them of the damage in time.
 

After the fuel is purified, it can be sent to the ship diesel engine through the fuel supply system.
In recent years, due to the use of high viscosity low quality fuel, its preheating temperature has been greatly improved.
A pressurized fuel system is developed to avoid vaporization due to high preheating temperature when heavy oil is used in high (700mm2 /s).
As shown in figure 5-2, between daily fuel tank and fuel cycle oil add a pump, ensure the fuel on diesel engine fuel injection pump inlet pressure is 800 kpa (circulation pump outlet pressure is 1 mpa), oil (circuit) cycle is 400 kpa pressure, prevent the fuel system in high temperature (150 ℃) occurs when the vaporization and cavitation.

Ii. Main equipment and functions.

1.The purpose of the heavy oil transfer pump is to remove the heavy oil from any heavy oil tank to the heavy oil settling cabinet for settling and settling.
Refueling between the heavy oil tanks;
In special case heavy oil tank can be refueled to outboard.
The transfer pump is generally used in gear pump or screw pump.
 

2.Heavy oil purification treatment equipment heavy oil purification usually adopt precipitation, separation and filtration purification treatment measures.
 

Sedimentation shall be carried out in a specially designed sedimentation tank (two sets of sedimentation tanks shall be set). At least 12h shall be precipitated in accordance with the relevant regulations.
To improve the effect of purification, the precipitation of heavy oil in the cupboard should be preheated to 50 ℃ ~ 60 ℃, and sludge can be added as appropriate dispersant and hydrophobic agent, to make the suspended impurities in oil is easy to precipitate.
The sedimentation tank should be discharged regularly.
The filtration is done by a plurality of coarse and fine filters in the system.
The core of purification treatment is centrifugal separation, and its main equipment is centrifugal oil separator.
The centrifugal oil separator will be introduced in the fourth section.
 

3.The pre-heating treatment of the atomizing heater and the heating temperature is an important measure to ensure the normal operation of the diesel engine.
Saturated steam should be used as a heating source in accordance with the relevant regulations.
Preheating steam pressure should not be more than 0.8 Mpa (the equivalent of saturated steam temperature 175 ℃), in case of heavy oil coke precipitation precipitation in the heater.
 

Atomizing heater is an important preheating equipment.
According to the requirement of good atomization, the viscosity of heavy oil should be reduced to 12mm2/S ~ 25mm2 /S.
In according to the viscosity of atomized spray heater preheating temperature, should also be increased by 10 ℃ to 15 ℃, to counter the injection pressure and cooling effect on the viscosity.
In order to avoid fouling immediately after the heating of preheating temperature shall not exceed 150 ℃.
Usually, in export furnish fuel atomization heater viscosity transmission device (not shown in figure 5-2) measuring the change of the fuel oil viscosity, and through some adjusting mechanism, such as pneumatic diaphragm regulating valves to adjust the steam valve opening, ensure that the fuel oil viscosity and setting of atomization viscosity.
 

3. Maintenance and management of fuel system.

1.Fuel loading and measurement, storage and loading of fuel is a very important work, should be particularly careful in the operation.

According to the ship to save oil brand and quantity, considering the transportation task of the next voyage, put forward by the chief engineer come on quantity and specification, after they agreed with the captain, apply for refueling, cable company departments.
After the company's reply, contact the shipping agent for specific refueling.
Before refueling, you should master the actual oil storage variety and quantity of each tank, and try to hold the oil compartment as far as possible.
After the oil tank and quantity are determined, it is agreed with the chief officer to ensure the balance of the ship.
The second tube check the loading pipe line, the correct switch valve, plug the deck outlet hole, and the key inspection of the potential leakage.
Before the pump is formally opened, the second tube shall check the quality and quantity of oil supplied by the supplier, take the meter reading and check the water content in the oil with the water test.
Both the oil supply and the oil-receiving party shall specify the contact signal, and the ship shall be the main party, and both parties shall carry out the signal.
At the same time, the supplier can determine the oil supply speed.
In loading oil, smoking and open fire should be prohibited in areas where oil and gas may spread.
Immediately after the pump is started, listen to the flow of the oil and air and confirm that the oil has entered the predetermined shipping space.
The filling personnel should stick to the position, strictly follow the operating procedures, master the loading schedule, prevent running, run, leak.
When loading the oil, the valve should be closed.
When removing the oil hose, the pipe should be sealed or other effective measures should be used in advance.
Re-measure the quantity of oil in each tank, check the actual quantity of fuel supply, ask for oil sample and seal it on the spot, so as to find out the problem.
In addition to loading the fuel tank, the oil tank should be measured, and the fuel storage capacity of the oil tank shall be measured at the beginning and end of each voyage.
It is necessary to pay attention to the difference of draught before and after the ship, and calculate the control deck.
In the storage of fuel, it is necessary to pay attention to the same oil of different brands as well as the same number of fuel in different refueling ports, which should be tested when necessary.
When two different types of oil to mix the use, the biggest problem is that their incompatibility, after mixing the incompatible situation occurs, often produces chemical reaction, a large amount of asphaltene precipitation, sludge, cause jams and host combustion bad, a lot of smoke.
It is difficult to run a diesel engine when it is serious.
 

2.Selection of fuel heating temperature.

Heating oil (especially heavy oil) is a very important job.
Most sea vessels are heated by steam to ensure the use of saturated steam for safety regulations.
The heating temperature varies with the use of viscosity.
Heating in the fuel tank is for the purpose of lightering. Therefore, it should be ensured that the fuel oil in the vicinity of the oil pipe is not solidified as the principle, and the oil tank is heated to 15-20oc, and the vicinity of the outlet is 35 to 40 OC.
In the settling cabinet, heat up to 50-70 O C (lower than the flash point) to improve the precipitation effect.
In order to improve the separation effect, the oil temperature should not be too low or too high. For heavy oil, the maximum temperature should not exceed 98 OC.
In the daily oil cabinet, the heavy oil temperature should be kept at 70-80 O C.
In order to ensure the proper temperature of the fuel injected into the cylinder, it is very important to heat the fuel before the injection pump.
For medium and low speed diesel engines, the heating temperature of the atomization heater should reduce the heavy oil viscosity to 12 mm2/ s-25mm2 /S.
Considering that the pressure increase will increase the viscosity and the temperature of the tube to the injector, the heating temperature should be increased by 10-15 OC, which is generally heated to 100-150 OC.
The heating temperature above can be adjusted by the amount of steam.
 

3.Ensure fuel cleaning

It is mainly in the operation to carefully master the precipitation, separation, filtration and other purification procedures.
Regularly clean the sedimentation tank and daily oil tank, keep the oil tank clean and clean the fuel filter regularly.
In particular, it is necessary to increase the number of cleaning times of residual and fuel filters.
The working condition of the filter can be judged according to the pressure difference before and after the oil flowing through the filter. If the pressure difference increases beyond the normal value, it indicates that the filter has become dirty and blocked and needs to be cleaned immediately.
If there is no pressure difference or less pressure difference, it indicates that the filter is damaged or the filter element is not assembled correctly, and the inspection should be disassembled immediately.
 

4.The system is easy to accumulate gas in the system, and the gas often accumulates at the high point of the system.
After the system has gas, the oil supply pressure fluctuates, and can not even supply oil to stop.
The oil tank is filled with breathable tubes, and the fuel supply system is a closed system. In normal operation, there will be no air entering the fuel supply system.
Most of the gas in the system is entered when cleaning the filter and the maintenance pipeline. It may also be entered during the shutdown process by the occasional clearance of the injection pump.
Therefore, after cleaning the filter and pipeline, attention should be paid to the oil - filled drive.
 

5.Correct operation when changing oil.

When the ship needs to be disassembled for a long time in the mooring or fuel piping system, the diesel engine should be converted to light diesel before the engine stops, so that the heavy oil in the piping and equipment should be cleaned.
In addition, when the diesel engine in the maneuvering state, in order to make the diesel engine with motor performance (especially the starting performance), also had better use light diesel oil, and should be used after sailing ship normal heavy oil, in order to improve the economy.
This kind of light and heavy oil conversion operation in diesel engine operation is called oil change.
The basic principle of oil change operation is to prevent the oil temperature mutation, so as to avoid the injection pump plunger tight or bite to death.
When the heavy oil is changed to light oil, first turn off the heating valve of the fuel atomizing heater, turn off the viscometer, then cut off the fuel oil, and connect the diesel oil.
The original fuel oil and the newly injected diesel are gradually diluted in the oil collecting cabinet.
Since the dilution viscosity decreases faster than the oil temperature, it does not need to be heated.
Finally, the fuel pipe, low pressure fuel oil transfer pump, fuel atomization heater, and oil return pipe are filled with diesel fuel for the next start.
Change by light oil into heavy oil: first, the fuel oil service tank must be heated to use state, slightly open at the same time make the diesel fuel spray heater temperature rise to more than 40 ℃, then cut off the diesel, turn on the fuel oil.
Heat the fuel temperature to the temperature of the fuel oil atomization requirement in the time of consumption of diesel fuel.
 

In recent years, the ship with low speed diesel engine has been adopted in the fuel system, which can make the fuel flow through the fuel injection pump and the flow of the fuel injector.
In normal use, the manufacturer requires the use of heavy oil as a single fuel, unless the diesel engine is expected to stop for a long time, otherwise it is not necessary to use light oil.